This invention relates monitoring systems for thermal energy storage (TES) plants, more specifically, an automatic monitoring system for TES plants wherein the system, when connected to a central control, acts as a pilot ice tank for the new and/or existing TES plants of all sizes and types to automatically and precisely monitor ice inventory to further improve efficiency and energy savings.
Immediate application of the invention relates to the capacity of the pilot ice tank to produce and relay to the central control of the TES information regarding available ice inventory in the TES on a realtime basis. This realtime ice inventory level tells the owner and/or operator of the TES how much cooling capacity is available in the main ice tanks at any given time.
Based on the information provided by the pilot ice tank, an owner or operator of the TES plant can create records to track ice inventory levels in the central control of the TES. These records will help the owner or operator to determine, despite variable ambient conditions and/or variations on water level on the main ice tanks, the best low and high ice inventory values to start and/or stop the operation of the chillers during an ice-making mode to achieve maximum efficiency of the TES operation. Maximum efficiency results in maximum energy conservation and minimum operational costs.
These records are also useful tools to corroborate and adjust along the useful life of the TES the best low and high ice inventory values to start and/or stop the chillers.
Realtime information regarding the ice inventory level in the main ice tanks is therefore essential to maximize the efficiency of the TES. Once the information is received at the central control, the ice inventory information is analyzed. Then, a decision is made as to whether to issue a command to either start the chillers during the ice making mode every time maximum efficiency ice inventory depletion level has been reached in the main TES ice tanks or to stop the chillers during the ice making mode when the maximum efficiency high ice inventory level has been reached on the main ice tanks of the TES.
Thermal energy storage (TES) plants are widely used as an energy conservation system in air conditioning and industrial processes. They have been used for cooling since the earliest days of mechanical refrigeration, initially and more specifically as an energy saving device in breweries. Recently, however, the concept has gained widespread exposure due the deregulation of energy rates and consequent changes in energy pricing policies. Utility companies are now using time-and-use rate schedules, attempting to encourage people to shift their electric demand to off-peak, low electrical demand periods of the day by raising daytime rates for energy consumption. Thus, in order to reduce energy costs, approximately five to ten thousand facilities in the United States and approximately five to eight thousand facilities in the rest of the world have switched to TES systems to air condition their buildings and to cool tools and manufactured parts in industrial processes.
The most basic TES cooling system is a chiller-based, closed loop system. Water is cooled by chillers during off-peak (less expensive) hours and stored in an insulated tank. During peak hours, the stored cool water is pumped to the air conditioning units in order to cool a facility. Thus, several benefits, both monetary and environmental, are achieved by using a TES system. More common TES systems accumulate ice instead of chilled water inside the tanks. Using ice in lieu of water allows the TES system to store the same amount of energy in a relatively smaller space.
Examples of facilities reaping the benefits of using TES systems include Florida Gulf Coast University in Fort Myers, Fla., which saves an estimated $11,000.00 per month on energy bills and the Centex building, a 180,000 square foot facility located in Dallas, Tex., which received a 99/100 rating by the Environmental Protection Agency's Energy Star Program in 2000.
A typical TES cooling system works by using an insulated tank (called a thermal energy storage tank) that contains a heat exchanger within the tank surrounded by water. During the off-peak hours (usually in the evening) the system is in the ice making mode, often referred to as the “off-peak” charge cycle. In this mode, a certain mixture of water and ethylene glycol is cooled by a chiller to a temperature below the freezing point of water and is circulated through the heat exchanger. Since the water/ethylene glycol solution is below freezing, the water surrounding the heat exchanger in the tank freezes. This process continues until a large percentage of the water in the tank is frozen solid. The percentage of water that should be frozen is specified by the tank manufacturer and determines the cooling capacity of each tank in terms of hours per cooling capacity (tons/hour).
During peak hours when energy costs are higher, usually during the day but always determined by hourly energy market costs, the process is reversed. The ice in the tanks thaws, thereby cooling the water-glycol solution now circulating through the air conditioning or industrial process system. The water-glycol solution then absorbs the energy from the building and its occupants or from an industrial process. This is known as the ice melting mode, also referred to as the “on-peak” discharge cycle.
Most current methods for monitoring, tracking and controlling ice inventory in the tanks of the TES are based on a calibrated electronic sensor permanently and hydraulically connected to the interior of the tank and installed inside a plastic tube externally attached to the tank. As the ice is made or thawed inside the tank, changes in the ice volume makes the water level in the tank go up and down, raising the water level as ice is made and lowering the water level as ice is thawed. As the water level changes inside the tank, the water level also changes inside the plastic tube at exactly the same pace and level as within the tank because the external plastic tube is permanently and hydraulically connected to the interior of the tank. Every change in water level inside the tank is therefore registered by the calibrated sensor inside the external plastic tube and converted by an electronic transducer into a digital or analog electronic signal sent to the TES central control as realtime data. The central control then displays the information as actual ice inventory percentage on the TES. the ice inventory information is used by the central control as previously described to start and/or stop the chillers in the ice making mode or as an indication of the cooling capacity available.
Another commonly used method to control the ice making process on a TES is by constantly monitoring the temperature of the water-glycol mixture returning from the tanks to the chillers. In this method, calibrated electronic sensors immersed in wells in the water-glycol mixture mainstream constantly read the temperature of the mixture and electronically send this information to the central control. The central control instantaneously and automatically translates the digital or analog signals from the sensors into Fahrenheit degrees. When the temperature of the mixture returning from the tanks equal a predetermined temperature setting on the central control, the chillers will stop. The central control settings to stop the chillers range from 26 to 28 degrees Fahrenheit. At that mixture temperature, the ice inventory in the tanks will be close to or at 100%.
Some external factors make these two monitoring systems inaccurate. The first system is inaccurate due to rainwater and high humidity affecting the internal water level, especially during the summer months. Despite best efforts to tightly seal the tanks, rainwater and humidity enter the tanks, causing the internal water level to rise. Subsequently, the level sensors in the plastic tubes will essentially be reading water levels not corresponding to actual ice inventory in the tanks. Thus, the sensors are relaying inaccurate information to the central control. During the dry season, water evaporation from the tanks will also lead to inaccurate ice inventory readings. Additionally, oftentimes the level sensors in the plastic tubes located outside the tanks go bad due to constant exposure to the elements.
The main causes for the inaccurate ice inventory level readings when using the second system are outdoor ambient temperature changes and temperature sensors failures. Inaccuracy in ice inventory level readings has a direct consequence: inefficiency.
Inaccuracy in ice inventory level readings risks not having air conditioning in a facility or no cooling available for an industrial process during the on peak hours of the day. In the occurrence of such an event, the owner has the option of running the chiller(s) during the on peak hours at great expense.
A third system measures the displacement of the heat exchanger and its supporting structure as the buoyancy of the ice lifts them up. Displacement is measured manually or electronically.
A fourth system positions a coil on springs and employs load cells to sense the uplifting force of the ice forming on the coils which are restrained from vertical movement.
Although accurate and reliable, the third and fourth systems include expensive, complicated to manufacture and difficult to install parts. Thus, cost, downtime and technical difficulties to adapt to existing TES have kept these systems out of the market.
Thus, there exists the need for a more accurate, economical and universal system and method to measure the amount of ice made and/or thawed on TES.
The relevant prior art includes the following patents:
Patent No.(U.S. unless stated otherwise)InventorIssue Date5,090,207Gilbertson et al.Feb. 25, 19925,467,812Dean et al.Nov. 21, 19955,678,626GillesOct. 21, 19975,390,501DavisFeb. 21, 1995JP359077254AOkada et al.May 2, 1984JP362141435ATakebayashi et al.June 24, 19875,139,549Knodel et al.Aug. 18, 19924,088,183Anzai et al.May 9, 19786,185,483 B1DreesFeb. 6, 20015,046,551Davis et al.Sep. 10, 19916,298,676Osborne et al.Oct. 9, 20026,415,615Osborne et al.Jul. 9, 2002